Kidney epithelial cells in short-term primary culture have been studied with regard to proliferative rate and expression on thec-fosprotooncogene. The experiments were performed on subconfluent renal proximal tubule cells isolated from infant and adolescent rats. Proliferation was determined by3H-thymidine autoradiography and nuclear content ofc-fosprotein by semiquantitative immunofluorescence. The basal proliferative rates in infant and adolescent renal proximal tubule cells were the same after 48 h of primary culture in Dulbecco's modified Eagle's medium with 10% FCS. Serum deprivation for 24 h caused a significant growth inhibition in both infant and adolescent cells. C-foswas expressed to the same extent in infant and adolescent serum-deprived cells. The rapid response to the addition of serum was markedly different in infant and adolescent cells. In adolescent cells, addition of serum led to a transient significant increase in the nuclear expression ofc-fosprotein, reaching a peak at 60 rain. No increase hic-foswas seen in infant cells. In adolescent cells, the rate of proliferation increased 11-fold and3H-thymidine labeling index reached 26.7 $$ 4.3%. In infant cells, the proliferative response to serum addition was significantly lower; the labeling index reached only 4.2 $$ 1.2%. It could be excluded that the attenuated response in infant cells was due to cell death or impaired metabolic function. The results imply that the principles of growth regulation change postnatally. (Pediatr Res 29: 263–267, 1991)